1. Field of the Invention
The present invention relates to a modulator circuit which includes the cascade connection, between the poles of a DC supply source, of a switching circuit and of an amplifier circuit and is adapted to modulate the amplitude of a carrier signal applied to a carrier input of said switching circuit with a modulating signal applied to a modulating input of said amplifier circuit, thereby producing a modulated output signal at a modulator output of said switching circuit.
2. Description of the Prior Art
Such a modulator circuit is already known in the art, e.g. from the book "Bipolar and MOS Analog Integrated Circuit Design" by A. Grebene, J. Wiley and Sons, New York 1984 and more particularly from pages 469 to 472 thereof. Therein the amplifier circuit is a so-called differential pair which includes two transistors whose emitters are coupled to a pole of the DC supply source via individual emitter degeneration resistances and a common constant current source, the bases of these transistors constituting the terminals of the modulating input. The switching circuit comprises two load resistances and two differential pairs each including two transistor switches with joined emitters and the bases of which are connected to terminals of the carrier input. The other pole of the DC supply source is connected to the collectors of the transistors of the amplifier circuit via two circuits each comprising one of the load resistances and the collector-to-emitter paths of two of the transistor switches, one of each differential pair. The purpose of the emitter degeneration resistances is to improve the linearity of the amplifier circuit. However, they have no effect on the distortion introduced in the modulated output signal by the limited speed of operation of the transistor switches driven by the carrier input signal.